Xan wrote:In a low orbit satellite loses energy due to friction of the atmosphere. Aerodynamic drag is equal to:

F = S * Cx * rho * v^2 / 2

Unfortunately this formula isn't valid at such low pressures. Once you get above 50km (~100 Pa), you're transitioning into a molecular flow regime. By the time you get to 100 km (~0.1 Pa) you're all the way there.

Ah, I apologise. I did some reading around and it seems you are indeed correct.

What does appear to be different in molecular flow (and quite drastically so) is the coefficient of drag. However I see that you have essentially isolated that by comparing to sputnik and instead only looking at the surface density.

Since Sputnik was essentially a sphere, and I would assume n-prize satellites are more likely to be cuboid, it would be interesting to see how they compare to the GOCE satellite, which was more angular, or some low altitude cube sats.

To be honest, I'm pleasantly surprised that drag isn't worse than it is, for N-class satellites. I'd have expected the difference between Sputnik and a nanosat to have been much greater than the calculations suggest...

Thank you for writing “N-class” satellites! I’ve said it before, but it bears repeating; by creating the N-Prize Paul has effectively defined the lower limit of true spaceflight -- orbital and suborbital nanosatellites carried aloft by nanolaunchers at minimal cost. The fact that aerodynamic drag in the “vacuum” of low-Earth space is a real factor in determining whether a tiny spacecraft might achieve nine revolutions or only eight is clear evidence of this. Fortunately that drag is just a very small factor.

Using expressions such as N-class or N-type spaceflight reinforces the recognition the N-Prize deserves for setting such “low” standards for us all! Thanks again, Paul.

I've recently been working on implementing atmospheric models into some orbit software to take account of drag, and thought I'd share this plot.

I used the same 20g and 20 cm^2 (ie: 1 g/cm^2) satellite and the US Standard Atmosphere 1976, and I gave it the slightly (but not overly) pessimistic drag coefficient of 2.4. The initial conditions were for an equatorial and circular 260km orbit, and it completed around 9.4 orbits.

I can relatively easily run more orbits if people are interested in seeing such things, and may run some more myself. So if anyone is curious to know how a certain size/mass/orbit combo performs let me know and I'll crunch the numbers.

Note: The Earth is not to scale (because it's huge), but the atmosphere and orbit track around it are. The blue part is my representation of the atmosphere and extends to 100 km (Karman line), but the full atmospheric model goes up to 1000 km.

OK, here's one more. This is the same 1 g/cm^2 Cd = 2.4 Satellite, but placed into the same orbit as Sputnik was. This is an elliptical orbit with perigee around 235 km and apogee at 960 km. The n-prize sat lasted for 165 orbits.

This is basically the kind of orbit that would have been required had the rules not been revised down from a lifetime of 99 orbits, to 9. Of course, a higher circular orbit would do, but those are so much harder to achieve compared to staying relatively low and burning until the fuel runs out.